The invention relates generally to a method for regenerating or replenishing an ink sheet that is used for heat-transfer printing and more particularly to a method and apparatus for electrostatically supplying replacement ink to depleted portions of an ink layer from which ink has been transferred during printing.
Conventional methods for regenerating a depleted ink sheet include direct ink layer regenerating in which hot-melt ink is melted and the melted ink is applied to a depleted ink sheet. An example of this method is described in SID 1985 Digest, pages 143-145. However, direct ink regenerating has drawbacks. Considerable time is necessary for warm-up to occur while the ink in a storage container is melted. A large amount of electricity is needed to maintain the replacement in a molten state. This method is also inefficient because replacement ink cannot be supplied selectively to only the depleted portions. Rather, the entire ink layer is continuously regenerated and mechanisms are required to maintain proper and constant ink layer thickness. Additional mechanisms are required to remove melted ink from the ink sheet after power is turned off and maintenance of the apparatus can be complicated.
A method of replenishing an ink sheet that uses powdered ink is described in Japanese Application No. 31332/88. Another is described in U.S. Pat. No. 4,467,332 in which powdered ink is transferred from the surface of an electrode to primarily depleted portions of the ink layer of the ink sheet. However, this method also has shortcomings because powdered ink will unintentionally adhere to undepleted portions of the ink sheet. Although the amount of ink adhering to the recorded and unrecorded portions of the ink layer can be substantially controlled, distributions of electric potential appear at the interface between depleted portions and undepleted portions of the ink layer. This phenomenon is referred to as the edge effect and adhesion of powdered ink at the vicinity of the interface will increase. This leads to an uneven ink layer and can lead to additional problems when attempting to adjust the thickness of the regenerated ink layer.
Accordingly, it is desirable to develop an improved method and apparatus for replenishing an ink sheet which avoids the shortcomings of the prior art.